hiho第九十三周

Eular质数筛法

#include <bits/stdc++.h>
#define maxn 1000010
using namespace std;
int mindiv[maxn] = {0};
int prime[maxn], prime_count = 0;
int main()
{
int n;

scanf( "%d", &n);
for (int i = 2; i <= n; i++) {
if (mindiv[i] == 0) {
prime[prime_count++] = i;
}
for (int j = 0; j < prime_count && i * prime[j] <= n; j++) {
mindiv[i * prime[j]] = prime[j];
if (i % prime[j] == 0) break;
}
}
printf( "%d\n", prime_count);
return 0;
}